1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a semiconductor device comprising packing members for protecting semiconductor elements and other electronic components from the external environment.
2. Description of the Prior Art
In general, a semiconductor device comprises a structure in which semiconductor elements and electronic components such as an integrated circuit (referred to hereinafter as an IC), transistors, diodes, capacitors, resistors, a hybrid IC, microcoils, etc. are provided on one substrate or more. A semiconductor device comprising such elements and components is normally of considerably small size and as a result, is liable to undergo physical or chemical changes due,to the external environment. In order to prevent such changes and assure stability and long life, a semiconductor device is normally provided with protecting means. Such protecting means is formed so as to wrap the semiconductor elements and electronic components, for the purpose of protection.
One conventional protecting means widely utilized is formed by an airtight sealing method, in which metal or ceramic is used. A notable increase of production of semiconductor devices in these days has led to the use of a resin sealing method utilizing organic materials, instead of the above mentioned airtight sealing method. With resin sealing, a transfer molding method is applied in which a molding material heated and softened in advance is compressed into a heated metallic mold by means of a press. As the molding material for use in this transfer molding method, epoxy resin or silicone resin mixed with a curing agent, a bulking agent, a flame resistant agent, a coupling agent and a coloring agent is utilized. In particular, a mixture composed principally of epoxy resin is utilized in many types of semiconductor devices, because this mixture of epoxy resin has various excellent characteristics.
In a semiconductor device provided with protecting means formed by the transfer molding method using as the molding material a mixture composed principally of epoxy resin, IC and other semiconductor elements and electronic components are completely sealed with resin. As a result, tips and bonding wires of the IC etc. are directly in contact with the resin and accordingly, such a semiconductor device provided with protecting means formed by the resin sealing method has excellent characteristics in moisture resistance, heat conductivity, shock resistance and the like, as compared with a semiconductor device sealed by an airtight sealing method using metal or ceramic.
However, a remarkably high degree of integration of circuits attained recently results in a demand for realizing a semiconductor device having a higher reliability with respect to the above stated characteristics and other characteristics. In order to satisfy such demand, it is necessary: (i) to improve moisture resistance; (ii) to prevent the electronic components from being broken due to curing stress of the sealing material, that is, a resin; (iii) to decrease soft errors in the semiconductor device, caused by X rays radiating from the sealing material; (iv) to prevent falling down of gold wires in the electronic components, which could be caused by a resin having a high viscosity at the time of resin sealing by a transfer molding method; and to take other measures. With a view to complying with these requests in a conventional semiconductor device comprising protecting means formed by a transfer molding method using a mixture of epoxy resin, the purity of resins or bulking agents has been enhanced and the types and mixing quantity of a catalyzer or a surface treatment agent as well as the manufacturing process for resin sealing have been re-examined, which has brought about an improvement of the performance to a certain degree. However, there is a limit in such improvement.
More specifically stated, concerning the improvement of moisture resistance as described above in (i), there are problems as follows. Deterioration due to moisture in a semiconductor device is caused principally by moisture penetrating through two paths, that is, moisture infiltrating a resin layer and moisture penetrating through a surface of contact between the resin layer and the lead wires. The moisture penetrating through these paths dissolves therein impurity contained in the sealing resin, so that the moisture itself is made alkaline or acid. This moisture reaches to the aluminum electrodes formed on the surface of the electronic component, causing corrosion in the aluminum electrodes. For this reason, in order to improve moisture resistance, efforts have been made to improve the purity of resins and bulking agents, re-examine the types and mixing quantity of a catalyzer or a coupling agent, improve the processing method for these materials, etc., and actually, an improvement of the performance has been realized degree. However, as far as a resin permeable by water is used, the moisture penetrating to the surface of the electronic component cannot be completely removed.
Damage or distortion in the electronic components, caused by curing stress of a sealing material, as described above in (ii), can be minimized by decreasing the difference between the coefficient of linear expansion of the material of an electronic component, e.g. a wafer for IC, and that of a resin used as sealing material. The decrease in the difference between the coefficients of linear expansion is generally made by adding a bulking agent such as silica into the resin. However, it is substantially difficult to make no difference exist between the coefficients of linear expansion of the resin and the electronic component by adding a large quantity of bulking agent into the resin. In addition, unfavorably, the bulking agent added for the purpose of decreasing stress contains thorium, uranium and the like, and radioactive rays emitted from these substances, particularly .alpha. rays, cause soft errors in the electronic component.